1. Technical Field
The present invention relates to a process for producing isobutylene and methanol. Particularly, the present invention relates to a process for producing isobutylene and methanol, in which methyl-tert-butyl ether is decomposed into isobutylene and methanol, isobutylene and methanol thus obtained are separated respectively thereby recovering isobutylene and methanol individually, the process being capable of suppressing contamination of methanol as an objective product, with water and tert-butanol and preventing losses of methanol and isobutylene.
2. Description of Related Arts
Methods of recovering isobutylene and methanol by decomposing methyl-tert-butyl ether into isobutylene and methanol and separating isobutylene and methanol respectively therefrom are known (e.g. JP47-041882B).
Further, U.S. Pat. No. 4,544,776 discloses that, in the case of etherifying C4-C7 iso-olefins with methanol, the etherification product(for example, methyl-tert-butyl ether) containing alcohol components such as methanol and a tertiary alcohol (e.g. tert-butanol) is extracted with an excess of water utilizing a washing column directly connected to the reactor as an aqueous extract; when the aqueous extract is distilled utilizing a distillation column, a tert-butanol-water binary azeotrope is taken overhead in this column and thus undesirably increases the water content in methanol to be recover; and when this methanol is reused for etherification, the water contained increases the formation of tert-butanol and thus the increase ultimately results in an additional xe2x80x9csnowball effectxe2x80x9d. As a solution to the above problem, the patent discloses a process wherein an alcohol component is extracted using an excessive amount of water so that a ratio of methanol/water to be extracted becomes 1:2 to 1:5, then the aqueous extract thus obtained is distilled to recover methanol from the top of the distillation column and a highly concentrated tert-alcohol is drawn as a side-stream to be retuned to the washing tower. However, the above process is non-economical since the ratio of water is excessively high, and the process has a drawback that tert-butanol formed by the etherification reaction is ultimately contained in product methyl-tert-butyl ether as one of impurities.
In addition, in a process of recovering isobutylene and methanol by decomposing methyl-tert-butyl ether into isobutylene and methanol and separating isobutylene and methanol respectively therefrom, tert-butanol is also usually contained in methyl-tert-butyl ether as a raw material, in an amount of 0.2 to 0.8 wt %. Depending on a decomposition ratio of methyl-tert-butyl ether, though 50 to 80% of tert-butanol is usually decomposed into isobutylene and water in the first step, undecomposed tert-butanol is distilled from the top of the distillation column in the methanol recovery step due to the residual tert-butanol, to thereby cause a problem that a water content in methanol is increased due to the azeotropy of tert-butanol and water.
An object of the present invention is to provide a process for producing isobutylene and methanol, wherein isobutylene and methanol are recovered by decomposing methyl-tert-butyl ether into isobutylene and methanol to obtain a mixture thereof, and separating isobutylene and methanol therefrom, the process being capable of suppressing a contamination of methanol as an objective product with both of water and tert-butanol and preventing from losses of isobutylene and methanol.
Namely, the present invention relates to a process for producing isobutylene and methanol comprising decomposing methyl-tert-butyl ether into isobutylene and methanol to obtain a mixture containing isobutylene and methanol, separating isobutylene and methanol respectively therefrom, thereby individually recovering isobutylene and methanol, the process comprising the following steps:
a first step of subjecting methyl-tert-butyl ether to decomposition in the presence of a solid acid catalyst to obtain a reaction liquid containing isobutylene, methanol, and, as by-products, a dialkyl ether and heavy components containing an isobutylene dimer and an isobutylene trimer;
a second step of washing the reaction liquid obtained in the first step with water to separate the reaction liquid into an oil layer and a water layer;
a third step of subjecting the oil layer obtained in the second step to distillation using a distillation column to obtain a fraction containing isobutylene and the dialkylether from the top of the distillation column and a fraction containing methyl-tert-butyl ether and the heavy components from the bottom of the distillation column; and
a fourth step of subjecting the water layer obtained in the second step to distillation using a distillation column to obtain a fraction containing methanol from the top of the distillation column, a fraction containing water from the bottom of the distillation column and a fraction rich in tert-butanol from a side cut of the distillation column, and then recycling at least a part of the fraction obtained from the side cut to the first step.